1. Field of the Invention
The present invention relates to a cap assembly and a secondary battery having the same, and more particularly, to a cap assembly preventing soot and degradation of a gasket thereof when a can is sealed using a laser beam, and a secondary battery having the same.
2. Description of the Related Art
In recent times, compact, slim and light-weight portable electric/electronic devices including cellular phones, notebook computers and camcorders are actively developed and produced. These devices have a battery pack to be operated in any places to which a power source is not provided. Examples of the battery pack include nickel-cadmium (Ni—Cd), nickel-metal hydride (Ni-MH) and lithium (Li) batteries, in an economical aspect, which are generally rechargeable secondary batteries.
Among them, the Li secondary batteries are widely used to the portable electric-electronic devices because of a three times higher operating voltage and a higher energy density per unit weight than the Ni—Cd and Ni-MH batteries. The Li secondary batteries are classified into a lithium ion battery using a liquid electrolyte and a lithium polymer battery using a polymer electrolyte according to the kind of the electrolyte used, or cylindrical, prismatic and pouch type batteries according to the shape of the battery.
The secondary battery generally includes an electrode assembly, a can accommodating the electrode assembly and an electrolyte which allows migration of lithium ions in the electrode assembly, and a cap assembly sealing the can. Here, the electrode assembly includes a positive electrode plate having a positive electrode collector to which a positive electrode active material is applied and a positive electrode tab electrically connected to a side of the positive electrode assembly. The electrode assembly also has a negative electrode plate having a negative electrode collector to which a negative electrode active material is applied and a negative electrode tab electrically connected to a side of the negative electrode collector, and a separator disposed between the both electrode plates.
For a prismatic secondary battery, the cap assembly generally includes, but is a bit different from other types of the Li secondary battery, a cap plate having a predetermined-sized terminal through-hole, an electrolyte inlet and a safety vent and coupled to the can. The cap assembly also has a terminal plate disposed under the cap plate and electrically connected to a negative or positive electrode tab of the electrode assembly. The cap assembly also has an insulating plate disposed between the cap plate and the terminal plate, an electrode terminal electrically connected to the terminal plate through the terminal through-hole of the cap plate, and a gasket insulating the electrode terminal from the cap plate.
Conventionally, in order to enhance the connection strength between the can and the cap plate, a method of welding the cap plate of the cap assembly to the can using a laser beam is used. However, in the secondary battery used in a small-sized electronic/electric device, the distance between a welding point of the cap plate and an outermost edge of the gasket of the cap assembly is not sufficiently large. Consequently, the gasket may become sooty or degraded as a result of the laser welding, and thus the secondary battery may not be tightly sealed, resulting in reduced reliability of the secondary battery.